The Challenges of Nanotech Energy Storage

Nanotechnology has important contributions to make to the efficiency of energy storage. Several nanotech research groups have demonstrated the potential of batteries with five to ten times the power density of currently available lithium-ion batteries. But don’t grab your car keys to go battery shopping. Several issues must be resolved before these techniques are incorporated into batteries in an affordable way.

Eliminate silicon cracking: Researchers at Stanford University have grown silicon nanowires on a stainless steel substrate (a structure on which a substance can be grown or deposited). Batteries built with anodes using these silicon nanowires have up to ten times the power density of conventional lithium-ion batteries.

The silicon nanowires eliminate the problem of silicon cracking that occurs when using bulk silicon. The cracking is caused by the swelling of silicon as it absorbs lithium ions while the battery is recharged and the contraction of silicon as the battery is discharged (when the lithium ions leave the silicon). Researchers found that the silicon nanowires swell and contract but don’t crack.

Overcome power density limits: The busy folks at MIT have developed a technique to deposit aligned carbon nanotubes on a substrate for use as the anode, and possibly the cathode, in a lithium-ion battery. Organic molecules attached to the carbon nanotubes help them align vertically on the substrate. The molecules contain many oxygen atoms that provide points that lithium ions can attach to.

This method could increase the power density of lithium-ion batteries significantly, possibly up to ten times. A battery manufacturer called Contour Energy Systems has licensed this technology and is planning to use it in their next generation of lithium-ion batteries.

Extend battery life: Another promising way to increase the power density of batteries is to incorporate sulfur in the cathode. The cathode is the electrode that the lithium ions move to when the battery is discharged. In lithium-sulfur batteries, the cathode is a combination of conventional carbon and sulfur.

The lithium ions attach to sulfur molecules and the carbon conducts electrons to and from the wires outside the battery. However, sulfur can dissolve from the cathode, limiting the battery’s lifetime.

Researchers at the University of Waterloo demonstrated that they could significantly reduce the loss of sulfur from the cathode by fabricating the carbon with many tiny nanopores filled with sulfur.

Various researchers have been trying for years to get lithium-sulfur batteries to work because the electrochemical reaction with lithium sulfur gives off more energy, which should make the power density of lithium-sulfur batteries about three or four times higher than lithium-ion batteries. The work of these researchers brings us closer to the day when lithium-sulfur batteries will become practical.